1. Field of the Invention
The present invention relates generally to a communication system using multi-level modulation/demodulation, and in particular, to a method and apparatus for rearranging a bit stream of a codeword sequence before modulation, and arranging an output of a demodulator in the form of an original codeword sequence.
2. Description of the Related Art
An IMT-2000 (International Mobile Telecommunication-2000) system or a UMTS (Universal Mobile Telecommunication System) system, a typical CDMA (Code Division Multiple Access) mobile communication system, uses multi-level modulation/demodulation in order to increase spectral efficiency. Here, the “multi-level modulation” refers to 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation) and 64QAM modulations as 2m-ary modulation having a higher modulation level than QPSK (Quadrature Phase Shift Keying) modulation. When generating modulated bits using the multi-level modulation, there occurs a difference in reliability between a plurality of bits constituting each modulated bit. Due to the reliability difference, the respective bits have different average bit error rates.
Meanwhile, when an encoder comprised of a plurality of system encoders, such as a turbo encoder, is used as a channel encoder, codeword bits from the system encoders are divided into a systematic part, a stream of bits with higher priority (importance), and a parity part, a stream of bits with lower priority. In the case where one information sequence is input to a plurality of constituent encoders, a decrease in a bit error rate of bits belonging to the systematic part of the codeword bits may result in a decrease in an average bit error rate and an average packet error rate of an information sequence output from a channel decoder. That is, when the codeword bits belonging to the systematic part with higher priority assuredly have higher reliability than the codeword bits belonging to the parity part with lower priority, it is possible to expect a performance improvement in terms of an error rate of an information sequence generated after decoding.